#include <stdio.h>

//https://www.nowcoder.com/practice/8f71f5670e6a45118d24d13868a2da9e?tpId=290&tqId=169881&ru=/exam/oj/ta&qru=/ta/beginner-programmers/question-ranking&sourceUrl=%2Fexam%2Foj%2Fta%3FtpId%3D290

int main() {

    int num = 0;
    int day = 0;
    int sum = 0;
    int count = 1;
    scanf("%d", &day);
    for (int i = 0; i < day; i++, num++) {
        if (num == count) {
            num = 0;
            count++;
        }
        sum += count;
    }
    printf("%d", sum);
    return 0;
}